Website Protection: Caching and CDN

Website Protection does more than filter traffic: it also speeds up your site by serving static files from a CDN. Here is what gets cached, how long it stays in the cache and how to clear it.

What is a CDN

A CDN, or content delivery network, is a system of servers located in different parts of the world. They cache static elements of your site: images, scripts, styles and other files. When a visitor opens the site, this data is loaded from the nearest CDN node instead of your main server. The site responds faster and performs better.

The CDN is enabled by default for all website protection services. You can manage it, including turning caching on and off, in the general service settings.

Content requirements

Files in the following formats are cached:

.js, .css, .ejs, .png, .jpg, .jpeg, .gif, .ico, .bpm, .svg, .svgz, .cur, .pict, .tif, .tiff, .eps, .ttf, .eot, .otf, .woff, .woff2, .swf, .mp3, .mp4, .ogg, .wav, .webm, .webp, .avi, .flv, .mov, .pls, .midi, .mid, .mkv, .m4v, .ts, .m3u8, .txt, .exe, .zip, .xz, .gz, .lzma, .7z, .rar, .class, .jar, .tar, .doc, .docx, .xls, .xlsx, .ppt, .pptx, .pdf, .rtf, .csv, .ps, .odt, .ods, .odp, .odg.

A server response is cached if:

• the request does not contain the Authorization header;
• the requested file has a format from the list above, OR the request contains an Accept header with a static content type (starting with image, audio, video, text/css, text/font), OR the request contains an If-Modified-Since or If-None-Match header;
• the response contains a Last-Modified, Expires or Cache-Control header;
• the response contains a Content-Length header whose value does not exceed 2048 megabytes.

The Accept header must match the regular expression ^(image|audio|video|text/css|font).

The ddg-cache-status response header shows where a file was served from: HIT means the CDN cache, MISS means your server.

Cache time and revalidation

Caching is controlled by the directives of the Cache-Control HTTP header.

How long a file stays in the CDN cache depends on its freshness, which is set in one of the following ways:

• if there is an s-maxage directive, the freshness period is taken from its value;
• otherwise, the max-age value is used.

If there are no such directives, freshness is estimated heuristically:

• from the difference between Expires and Date (if Date is later than Expires, freshness is zero);
• without Expires or Date, 10% of the difference between Date and Last-Modified;
• without Date or Last-Modified, 5 minutes.

Heuristic freshness always ranges from 5 minutes to 10 days.

A file is revalidated when its lifetime expires. By default, the lifetime equals the freshness, but some directives change it:

• min-fresh: revalidate when min-fresh seconds remain before the object’s lifetime ends. This shortens the file’s actual lifetime.
• max-stale: keep serving the file from the cache for max-stale seconds after its freshness expires. This extends the file’s actual lifetime.

How to clear the cache

1. Log in to your client area, open “Product/Services” and click “Site protection”.
2. Select your service and click “Clear cache” on the toolbar.

Right after clearing, the site may load slower than usual until the required files are cached again.